Device for determining the residual liquid content of solids cakes in centrifuges

ABSTRACT

A device for determining the residual liquid content of solids cakes in a centrifuge basket. The device includes two electrodes protruding through a wall of the basket and into the solids cake; a conductivity or capacitance measuring device ( 13 ) measuring the conductivity of the solids cake between the electrodes; and evaluation and control units contactlessly receiving data from the measuring device to affect the operation of the centrifuge.

This is a 371 of PCT/EP02/11619 filed 17 Oct. 2002 (international filingdate).

The invention relates to a device for measured value acquisition,processing and transmission in centrifuges for the separation of liquidsand solids, in particular skimming centrifuges.

BACKGROUND OF THE INVENTION

Skimming centrifuges with vertically or horizontally driven baskets areknown. It is also known that, in the case of poorly flowing moistsolids, the solids discharge takes place by means of screws instead ofby means of chutes.

There are known methods, specifically ultrasonic and infrared measuringmethods, which determine the degree of residual moisture of the solidscake in a skimming centrifuge during the dewatering operation by meansof contactless measurement. The reflection of the pulses takes place onthe surface of the filling, so that only the degree of residual moistureof the surface of the filling is determined.

With ultrasound, measurement is carried out, for example according toDE-A-19 716 128, in such a way that the echo delay of an ultrasoundpulse between the ultrasound transmitter, the impinged surface of thefilling and the ultrasound receiver is measured and the degree ofresidual moisture is determined by the evaluation of the quality of theecho of a reflected ultrasound pulse. Measuring errors are caused bydifferences in the nature of the surface, vapors, mist and dust.

According to laid-open patent application EP-A-891 814, infraredradiation is used for measuring the degree of moisture by the radiationintensity of a light pulse which is reflected from the surface of thefilling being measured and the degree of residual moisture beingdetermined by the evaluation of the change in the radiation intensity.Measuring errors are caused here by differences in the nature of thesurface and by differences in the color of the filling medium.

It has until now been necessary for a skimming centrifuge to be operatedon the basis of empirical values, according to which it is assumed thatthe solids cake has reached a specific residual moisture after aspecific dewatering period, whereupon further working steps can then becarried out, such as for example the skimming discharge of the solidscake. In the case of these empirical values, however, it is necessary totake into account different filtration rates, which are produced forexample when there are differently compacted base layers or differentsuspension characteristics, in particular solids size distribution.

After centrifuging off surplus liquid as a filtrate, the remainingsolids cake is dewatered, the degree of residual moisture depending onthe dewatering kinetics. The degree of residual moisture varies over theheight of the solids cake. The layer of the solids cake that is nearestthe basket shell, varying in its height and dependent on the fillingmedium, is saturated with liquid. The layer of the solids cake lyingover that is saturated at the beginning of the dewatering operation andhas moisture removed to an extent depending on the dewateringkinematics. At any time during the dewatering, the degree of residualmoisture at the surface of the filling is less than or equal to that ofthe remaining solids cake and is not necessarily always in the samerelationship with the degree of residual moisture of the remainingsolids cake.

The invention is therefore based on the object of designing a device ofthe type stated at the beginning in such a way that the degree ofresidual moisture can be determined in a way in which the product iscontacted and the measurement takes place continuously during thedewatering operation and over the entire height of the solids cake or alarge proportion of the height of the solids cake, in order to be ableto restrict the operating times of the solids-liquid separating deviceto the amount necessary in each case.

SUMMARY OF THE INVENTION

This object is achieved by a device for determining the residualmoisture content of solids cakes in centrifuges which at least comprisesat least two sensors, in particular electrodes, and a measuring devicewhich is connected to the sensors and is provided for measuring theresidual moisture, in particular a conductivity or capacitance measuringdevice, the sensors being arranged spaced apart from one another in theregion of the centrifuge basket.

DETAILED DESCRIPTION

The sensors are preferably provided in the edge of the basket.Consequently, it is possible for the dewatering over the cross sectionof the solids cake to be monitored.

Furthermore, the measuring device is preferably provided in the edge ofthe basket.

The sensors are particularly preferably electrodes of a materialselected from the series: corrosion-resistant steel, platinum, gold,nickel and copper. Since the electrodes are contacted by the product,chemically inert materials are preferred.

In a preferred configuration, the measuring device is connected to atelemetry unit, which transmits measuring data contactlessly from thecentrifuge basket to the outside.

Most particularly preferred is a variant of the device in which thetelemetry unit contains a transmitter, arranged opposite which in theregion of the centrifuge housing is a receiver. By this means,contactless data transmission from the centrifuge basket is possibleduring the operation of the centrifuge.

In a preferred configuration of the device, the receiver is connected toan evaluation unit and a control unit for controlling the operation ofthe centrifuge.

In a particular form of construction, the measuring device for sensingthe moisture is operated by means of an external voltage supply,arranged partly outside the centrifuge basket. A permanent voltagesupply and uninterrupted operation is hereby ensured, in particular inthe case of a measuring device provided on the rotating basket.

It is particularly preferred for the voltage supply to comprise ahigh-frequency transmitter with a transmitting coil and an AC/DCconverter with a receiver coil, the AC/DC converter being electricallyconnected to the measuring device on the basket and supplying thepossibly required DC voltage for the measuring device.

For protection from corrosion and attack by the solids and liquidstreated in the centrifuge, in a further preferred configuration of thedevice the voltage supply arranged in the region of the centrifugebasket, the measuring device and if appropriate the telemetry unit arearranged such that they are closed from the surroundings in a ring,which consists of a non-metallic material, in particular of glass-fiberreinforced plastic.

A curable plastic, in particular epoxy resin or phenolic resin, is usedparticularly preferably as the material. The electronic components onthe basket are embedded in this material.

It is particularly preferred for an additional supporting ring, which isfirmly connected to the edge of the basket, to be provided around theouter circumference of the ring.

The ring, the supporting ring and the transmitting and receiving unitsmay be connected releasably or unreleasably to the centrifuge basket orthe centrifuge housing.

The device according to the invention can be used in all centrifuges forthe separation of liquids and solids, in particular in skimmingcentrifuges or in other filtering centrifuges, in particular horizontalfilter-bag centrifuges and pendulum centrifuges as well as centrifugaldryers and screen centrifuges, in particular pusher centrifuges andscreen-conveyor centrifuges.

Centrifuge dryers are used for the separation of liquids and solids withmechanical and thermal dewatering.

The subject matter of the invention is therefore also a centrifuge, inparticular a skimming centrifuge, filtering centrifuge, centrifuge dryeror screen-conveyor centrifuge, having a measuring device according tothe invention.

The sensing and interpretation of the resistance or conductivity of thesolids cake allows the device according to the invention to be used todetermine the following:

-   The porous filter cake is saturated with liquid (a low resistance or    high conductivity is characteristic).-   The porous filter cake has been dewatered to a certain extent (an    increased resistance is characteristic, the conductivity decreasing    nonlinearly, but perceptibly and reproducibility as dewatering    progresses).

The device is capable of continuously determining the degree of residualmoisture by the evaluation of the conductivity of the solids cake over afilling height of several centimeters. When the desired degree ofresidual moisture is reached, the solids cake is discharged by means ofthe skimming device and a new processing cycle is commenced byintroducing suspension. The possibility of establishing the degree ofresidual moisture at the actual time allows the solids cake to bedischarged with the respectively desired residual moisture after theminimum possible dewatering period in each case.

Also mentioned as alternative suitable measuring devices for the sensingand interpretation of the degree of residual moisture are measuringunits with capacitors for capacitance measurement, which can beintegrated in the centrifuge basket in place of the conductivitymeasuring device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below by way of example on thebasis of the figures, in which:

FIG. 1 shows a skimming centrifuge with a measuring device according tothe invention in cross section as a diagram.

FIG. 2 shows an enlarged detail A from FIG. 1.

FIG. 3 shows a partial view of the edge of the basket with the measuringdevice, telemetry and power supply.

FIG. 4 shows a diagram to explain the processing cycle, in which thecurrent proportional to the conductivity is plotted against time.

EXAMPLES

An exemplary embodiment of a centrifuge in which a device according tothe invention is integrated is explained on the basis of FIG. 1, whichshows the section through a filter centrifuge in a schematicrepresentation. According to this, a filter basket 1 is mounted in aclosed centrifuge housing 2 and is connected to the drive (not shown).The filter basket 1 comprises a basket shell 3, containing a screenarea, and also a basket base 4 and a basket edge 5. The basket shaft 6bears the basket base 4 and is mounted in the centrifuge housing 2. Inan alternative form of construction which is not shown, the filterbasket 1 has a closed basket shell with a filtrate run-off via thebasket base.

Protruding into the filter basket 1 is a skimming device 7, which ispivotably mounted in the centrifuge housing 2. Furthermore, a fillingtube 8 for the product suspension and a washing tube 9 for one or morewashing liquids-protrude into the filter basket 1.

During the operation of the centrifuge, the suspension introduced intothe filter basket 1 via the filling tube 8 is distributed over thebasket shell 3 and separates into a solids cake 10 comprising particlesof solid matter, and a liquid filtrate. The said filtrate penetrates thesolids cake 10, is centrifuged off outward and drained away via thefiltrate outlet 11, or in the configuration not shown with a closedbasket shell 3 via the basket base 4, out of the centrifuge housing 2.The washing liquid introduced into the filter basket 1 is distributedover the remaining solids cake 10, penetrates the solids cake 10, iscentrifuged off outward and drained away via the filter outlet 11 fromthe centrifuge housing 2. The solids cake 10 is dewatered bycentrifuging off the pore liquid for a shorter or longer period of timeand discharged at intervals from the filter basket I by means of theskimming device 7.

The moisture measuring device is a conductivity measuring device,comprising the spatially separate electrodes 12 a, 12 b (shown in FIG.2), which are fixed to the edge of filter basket 1 and the ends of whichprotrude into the solids cake 10, and the measuring signal of which isconverted in a measuring transducer 13 and fed to a telemetry unit 14(shown in FIG. 3) operated at high frequency. The devices are suppliedwith DC voltage via an AC/DC converter 15 (shown in FIG. 3). Thecomponents 13 to 15 are integrated in a ring 16 made of phenolic resinreinforced with glass fibers, which is fastened on the edge of thebasket 5 and follows the movement of the filter basket 1.

Also integrated in the ring 16 over the circumference is a copperreceiving coil 17, via which an AC voltage is induced by a transmittingdevice 18 which includes a number of copper transmitter coils 19 and issupplied with power via a generator (not shown) being located on thecentrifuge housing 2.

To relieve the ring material of centrifugal forces, the ring 16 issurrounded by a steel supporting ring 20 connected to the edge of thebasket 5.

For the transmission of the measured signal, the measured value sent bythe transmitter antenna 21 of the transmitter of the telemetry unit 14is received by a receiver 22 and passed on to a measured value unit (PC)not shown here.

In particular if there is inadequate dimensioning of the gap between theouter surface of the edge of the basket 5 and the centrifuge housing 2,the units 16 and 18 may alternatively be integrated in the edge of thebasket 5 and the centrifuge housing 2, respectively.

An operational test which was carried out with the invention isexplained on the basis of a processing cycle diagram (see FIG. 4) of theskimming centrifuge based on the conductivity of the solids cake, andthe advantage of the device according to the invention becomes clear.

In the processing cycle VZ1, the suspension is fed to the filter basket1 in the time period (a) to (b), whereby the conductivity of the solidscake 10 rises to a maximum. The layer of liquid over the solids cake 10is centrifuged off to the outside and the operation of dewatering thesolids cake 10 commences between (b) and (c), with the result that theconductivity clearly drops. At (c), the rotational speed of the filterbasket 1 is raised and the dewatering operation is continued until (d),after which the solids cake 10 is discharged by skimming between (d) and(e) and a new processing cycle begins at (f). In the processing cycleVZ2, the operation of dewatering the solids cake between (h) and (j) isshorter than in the processing cycle, VZ1 between (c) and (d), with theconsequence that the conductivity or residual moisture of the solidscake is higher.

The residual moisture of the discharged solids cake can be checkedoffline on the basis of a sample of solid matter and compared with themeasuring signal at the time of the skimming discharge operation at (d)and (j).

1. A device for determining the residual liquid content of solids cakesin a rotating basket of a centrifuge, said device comprising: at leasttwo electrodes as sensors, disposed to protrude, spaced apart from eachother, through a wall of said rotating basket and into a solids cakewhich accumulates in said basket during operation; a conductivity orcapacitance measuring device (13) connected to the electrode sensors tomeasure the conductivity of the solids cake between said sensors and todetermine the residual liquid content thereby; and evaluation andcontrol units positioned apart from said basket and configured tocontactlessly receive data from said measuring device and, in responsethereto, affect the operation of the centrifuge.
 2. The device asclaimed in claim 1, wherein the basket comprises a basket shell,containing a screen area, a basket base and an edge, and the electrodesare provided in the edge of the basket.
 3. The device as claimed inclaim 1, wherein the measuring device is provided in the edge of thebasket.
 4. The device as claimed in claim 1, the electrodes are of amaterial selected from the group consisting of stainless steel,platinum, gold, nickel and copper.
 5. The device as claimed in claim 1,wherein the measuring device is connected to a telemetry unit (14),which transmits measuring data contactlessly from the centrifuge basket(1) to a receiver.
 6. The device as claimed in claim 5, wherein thebasket of the centrifuge is mounted in a centrifuge housing, and thereceiver is provided on the inside of the centrifuge housing.
 7. Thedevice as claimed in claim 6, wherein the receiver is connected to saidevaluation and control units for controlling the operation of thecentrifuge.
 8. The device as claimed in claim 1, wherein the measuringdevice is operated by an external voltage supply, arranged partlyoutside the centrifuge basket.
 9. The device as claimed in claim 8,wherein the voltage supply comprises a high-frequency transmitter with atransmitting coil and an AC/DC converter with a receiver coil, the AC/DCconverter being electrically connected to the measuring device.
 10. Thedevice as claimed in claim 8, wherein the voltage supply, the measuringdevice and a telemetry unit which transmits measuring data contactlesslyfrom the centrifuge basket to a receiver are arranged in a ring, closedfrom the surroundings, which ring is comprised of a non-metallicmaterial.
 11. The device as claimed in claim 10, wherein thenon-metallic material is a curable plastic.
 12. The device as claimed inclaim 11, wherein said curable plastic is an epoxy resin or a phenolicresin.
 13. The device as claimed in claim 10, wherein the basketcomprises a basket shell, containing a screen area, a basket base and abasket edge, and an additional supporting ring is provided around theouter circumference of the ring and is connected to the basket edge. 14.The device as claimed in claim 10, wherein said non-metallic material isa glass-fiber reinforced plastic.
 15. A skimming centrifuge, filtercentrifuge, centrifugal dryer or screen centrifuge comprising ameasuring device according to claim 1.